The current practice of solar-powered intermittent absorption refrigeration is exemplified in U.S. Pat. No. 4,744,224. This technology is simple, robust, and reliable. It meets the needs of lesser developed countries by being locally manufacturable and by producing ice at about one-tenth the cost of production by photovoltaic refrigerators, for ice capacity in the range of 10 to 1000 kg per sunny day.
Nevertheless, there still remain two limitations in the current practice of solar absorption refrigeration which have limited its spread. As with all solar technologies, high first cost is a problem. Any measures which would either increase the solar aperture or increase the overall collection efficiency without increasing cost would have the beneficial effect of lowering the first cost per unit of ice produced.
Secondly, the inherent functioning of solar intermittent absorption refergerators is to produce ice at night, which requires evaporator temperatures on the order of -10.degree. C., and then use stored ice by day to keep the cold box at slightly above 0.degree. C. In other words, the evaporator region inherently cycles between about -12.degree. C. and about +4.degree. C., depending on insolation and insulation. Clearly it would be possible to incorporate a separate thermostatted compartment cooled by storage ice which maintains a relatively constant +4.degree. C., and that would be useful for many refrigeration applications. However, there is another category of applications which require a relatively constant -20.degree. C. This is the temperature of the frozen food section of most domestic refrigerators, i.e., the "freezer compartment." Examples of commodities which require this level of refrigeration for long term storage include oral polio vaccine, measles, and yellow fever vaccines. Although conventional intermittent absorption cycles could easily be adjusted to yield -20.degree. C. at night, at some loss in efficiency, they have no practicable mechanism for maintaining that temperature by day.
Multiple-staged absorption cycles are well-known in the art, especially for continuous cycles. See for example U.S. Pat. Nos. 4,402,795 and 4,475,361. Some previous work has also been done on intermittent cycles with multiple stages, for example the technical article by A. Mani and A. Venkatesh appearing at p. 271 of Vol. 26 No. 3/4 1986 Energy Conversion and Management, entitled "A Two Stage Intermittent Solar Refrigeration System--Evaluation of Salient Parameters". In that article, a two-stage generator and absorber configuration is disclosed which enables use of much lower heat source temperatures (approximately 70.degree. C.), albeit at much lower Coefficient of Performance.
The capital cost problems relating to aperture size and collection efficiency stem from two constraints. First is the sidereal motion. The elevation angle of the sun at solar noon changes by 46.5.degree. through the course of the year. At three hours either side of solar noon, the change is about 5820 . Secondly, the inherent functioning of the intermittent absorption refrigeration cycle requires average temperatures on the order of 50.degree. C. above ambient, and afternoon peak temperatures some 15.degree. C. higher. The collection efficiency of flat plate collectors is simply too low at those temperatures.
It is known that as the collection temperature increases, a concentrating collector (solar aperture larger than solar target) becomes more efficient than a flat plate collector. The decreased loss due to heat leakage from the smaller target offsets the increased loss due to reflections. In the technical article "Low Concentration CPC's for Low-Temperature Solar Energy Applications", February 1986, Journal of Solar Energy Engineering, Vol. 108 p. 49, J. M. Gordon shows that a truncated CPC with acceptance half angle of 30.degree. becomes more efficient than a flat plate collector at 21K above ambient for a concentration ratio (CR) of 1, and at 36K for a CR of 1.5.
Clearly at the temperatures required for solar absorption refrigeration some degree of concentration is appropriate. However, just as clearly the cost and reliability constraints eliminate any use of automatic tracking concentrators.
Winston (U.S. Pat. No. 4,002,499) has shown that with a full CPC geometry the concentration ratio achievable from a stationary collector is 1/sin .theta., where .theta. is the acceptance half angle. Unfortunately the full CPC geometry is very wasteful of reflective material--much of it is shaded for much of the year. When the CPC is truncated to avoid shading, the CR attainable at a given acceptance half angle is much lower. If the aperture of a truncated CPC is increased to get more CR, the acceptance angle decreases, thus either missing some sun or requiring tracking.
It is known to increase the solar aperture by adding one or more hinged reflectors to an array, where the hinges are seasonally adjusted. For example, U.S. Pat. No. 4,371,623 discloses addition of hinged flat plate reflectors to a flat plate collector.
What is needed, and one object of this invention, is a solar energy collector which achieves the advantages of a stationary truncated CPC collector without the attendant disadvantage of low CR. Preferably any required seasonal repositioning of such a device would readily be accomplished by one person. Also desirable objectives are that the same geometry be applicable at different latitudes, and that the collector be acceptably storm-resistant.
A second needed improvement, and object of this invention, is a simple add-on to a solar intermittent absorption refrigerator which would allow maintenance of continuous -20.degree. C. temperature, preferably without requiring an additional generator.